My 1995 non Tacoma truck with a V6 and automatic transmission has 83000 miles. I have lost forward and reverse. The engine will rev freely but the truck will not move when put in gear. This happened suddenly when attempting to leave an intersection after a one hour drive on the freeway. No warning! However, 4WD low works fine! My mechanic has checked for stuck valves and still does not know what the problem is. No engine warning lights have come on. He is baffled. What is wrong with my truck?